Consider using command-line git client for fetching instead of libgit2

[rtimush]

Similarly to what Cargo does when net.git-fetch-with-cli is enabled.
This would make protofetch use the "system" authentication configuration, eliminating the need to provide protofetch-specific GIT_USERNAME/GIT_PASSWORD.

[jeanbza]

Yes, please do this. This was several years worth of issues over at jj-vcs/jj (ex jj-vcs/jj#5228) and resulted in subprocess spawning approach which has dramatically increased its capability.